The conventional formulation of lubricating oils addresses a number of problems relative to engine crankcase performance and attempts to improve significantly the properties of the normally employed mineral and synthetic oils. In a typical fully compounded mineral lubricating oil some 5 to 15 wt. % of the total composition will consist of additive components imparting improved viscosity, viscosity index, oxidation stability, sludge dispersancy, wear protection, acidity control, and the like.
In recent years more severe Federal standards have been imposed on engine performance in terms of control of combustion emissions and improved energy utilization, measured in terms of miles per gallon. This has resulted in increased demands and stresses upon automotive engines, particularly upon smaller engines at their relatively high operating speeds. For example, particular phosphorus compounds are normally introduced as a component of automotive engine oils to provide wear protection for engine surfaces and parts. However, even at the low levels required for adequate wear protection there are deleterious effects on exhaust system catalytic converters and on the resulting emissions. Similarly, such phosphorus compunds also affect the sensors developed for control of fuel/air ratio in modern engines.
Fuel economy is effected by lessening the friction between moving parts in a combustion engine, particularly by boundary film lubrication. It is often found that desirable anti-wear and friction modification properties do not occur together, probably because of differing sensitivities to chemical components present in the additives in the oil.
To achieve an optimized formulation of lubricating oil, the phosphorus content of the oil should be reduced as much as possible. Presently, the phosphorus is generally present at a level of 0.10-0.12 wt. % in the form of a zinc dialkyldithiophosphate (ZDP). Accordingly, there is a need for a non-phosphorus additive possessing anti-wear properties to permit a significant reduction in the phosphorus content of the lubricating oil. Additionally, such an additive should desirably impart significant friction-reducing properties.
Friction-reducing additives, based on fatty acid amide derivatives have been disclosed. In U.S. Pat. No. 4,208,293, the friction modifier is an amide, based on a fatty acid and diethanolamine. In related U.S. Pat. No. 4,201,684, corresponding amides and/or esters are sulfurized. More recently, U.S. Pat. No. 4,389,322 has employed borated amides and U.S. Pat. No. 4,512,903 has employed hydroxy-substituted fatty acids for reducing friction.
One example of an anti-wear additive is disclosed in U.S. Pat. No. 4,380,499 where a sulfurized triglyceride, transesterified for improved oil solubility, is employed.